Some Basic Rules of Thumb:
Know your level of play - whether you are self rated or have an NTRP Rating, you should know where you fall as a player.
BEGINNER (NTRP 1.0 - 2.5) If you're a beginner just starting out, you'll want a racquet that helps you return the ball with some power. Because you're developing technique and strokes, a racquet with a larger headsize will be forgiving when you don't hit the ball in the sweetspot.
INTERMEDIATE (NTRP 3.0 - 4.5)
If you're an intermediate player, you will be able to rally with moderate power and consistency, and you now have a good all-around game. You'll want racquets that can provide some additional power to your own without sacrificing control. You have experience playing matches and have developed the ability to put together points. You'll want a racquet that'll magnify the part of your game that is your weapon, whether it be a big forehand or serve.
ADVANCED (NTRP 4.5 - 7.0)
If you're an advanced player, you have a fully developed game and extensive experience playing competitive matches. Because you can consistently hit the ball hard when you want to or ease up on the power for touch, you'll be looking for racquets that build upon your strengths.
Make sure to educate yourself on subjects ranging from technologies and customization to how to measure your grip size, strings and much, much more. Your local tennis shop or pro you might be taking lessons with, can also help with this.